1. Field of the Invention
Compounds which are esters of phosphorus acids, wherein such compounds contain at least one P--O--C linkage, three phosphorus atoms, and wherein at least two of the phosphorus atoms are linked to each other by a carbon containing radical in which each phosphorus is attached directly to at least one carbon atom. The compounds within the scope of this invention are also flame retardants for polyurethane polymeric compositions.
2. Description of the Prior Art
During the past several years, a large number of flame retardants have been developed for use with an almost equally large number of flammable materials. Cellulosic materials (such as paper and wood) and polymeric materials (such as polyolefins, polyurethane, and polystyrene) are just two examples of materials for which flame retardants have been developed. For any class of flammable materials, such as synthetic type polymers, those skilled in the art have long been aware that some flame retardant additives are more effective in polymers and polymeric compositions than other flame retardant additives. This is because the efficacy of any flame retardant in polymers or polymeric compositions is measured not only by the flame retardant capability of the additive but also by the ability of the additive to improve or modify, or at least not to detract from, other physical or mechanical properties of the polymer or polymeric composition. The mere fact, therefore, that most flame retardants contain halogen and phosphorus atoms does not assure that any given halogenated or phosphorus-containing compound will impart usable flame retarding characteristics to all or even to any polymeric system. Furthermore, as those skilled in the art have improved the flame retardancy of many polymeric materials, they have been simultaneously required to provide the necessary flame retardancy with a minimal effect upon other properties of the polymer such as the light stability, processability, and flexural, tensile and impact strengths. Also, it has been the desire of those involved in the art of flame retardants to provide flame retardants having a durable lasting effect. Balancing all of the foregoing considerations and thereby developing polymeric compositions with good flame retardant characteristics as well as a satisfactory balance of other properties is, consequently, a task which has in the past and presently continues to require the exercise of a high degree of inventive skill.